Malignant prostate adenocarcinomas, with metastases, elevated serum acid phosphatase, have been observed thus far in 8 aged germfree Wistar rats. Three tumor lines have been transplanted to random-bred germfree and conventional rats: in vivo and in vitro, two of them are mixed epithelial and connective tissue types and the third is medullary (undifferentiated) type. All of them spread from the subcutaneous implant site to lymph nodes to the lungs. We seek to develop these tumors as model systems for prostate carcinoma. The tumors will be examined for etiology, characteristics of tumor cells as regard host- tumor interaction, the nature of metastatic process, and responses to selected hormone agents.